This invention relates to an eye refractive power measuring apparatus for detecting refractivity of an eye to be tested by projecting a measurement target image to the retina thereof and detecting a focussing state of the measurement target image.
In a conventional eye refractive power measuring apparatus, it has been heretofore required to properly set the measurement optical axis of a measuring optical system with respect to the eye to be tested, to reduce errors in the measurement result. In order to avoid the occurrence of such errors and improve measurement accuracy, an arrangement has been made such that an alignment of the measurement optical system with respect to the eye to be tested is effected before the eye to be tested is measured, by projecting a measurement flux of rays toward the eye to be tested from the measurement optical system.
For example, as shown in FIG. 1, in the conventional apparatus, there is provided an alignment target projection system 4 for projecting an alignment target flux of rays toward a cornea 3 of the eye 2 to be tested. The alignment system 4 is separate from a measurement optical system 1. An alignment target plate 5 (see FIG. 2) is illuminated by an illumination light source 6, the light transmitted through a circular pin hole 5a of the alignment target plate 5 is converted into a parallel pencil of rays and guided to the eye 2 to be tested as an alignment target flux of rays through half mirrors 8 and 9 to form a virtual image on the cornea 3. Reflection rays forming the virtual image on the cornea 3 are guided to an imaging lens 12 through the half mirrors 8 and 9, a relay lens 10, and a half mirror 11. An alignment target image is imaged on a photosensitive surface 14 of a photosensitive element 13 by the imaging lens 12 together with an anterior portion image, of the eye. A reference pattern (see FIG. 3) is illuminated by an illumination light source 15 with the light transmitted through a circular cross pattern 16a thereof being converted into parallel pencils of rays by a projection lens 17, and guided to the imaging lens 12 as a reference pattern flux of rays through the half mirror 11 and imaged on the photosensitive surface 14 of the photosensitive element 13 as a reference pattern image by the imaging lens 12. The anterior portion image, the alignment target image and the reference pattern image are converted into an electrical signal by the photosensitive element 13. The anterior portion image 19, the reference pattern image 20 and the alignment target image 21 are displayed on a display surface 18 of a television monitor as shown in FIG. 4. Positional alignment of a measurement optical axis of the measurement optical system within a plane intersecting the measurement optical axis at right angles is then verified by confirming the image displayed on the screen. At the same time, a distance in the optical axis direction or working distance of the measurement optical axis with respect to the eye 2 to be tested is adjusted based on the sharpness of the alignment target image 21. In FIG. 1, reference numeral 22 denotes an objective lens of the measurement optical system 1.
However, the above-mentioned conventional apparatus has presented crucial disadvantages or drawbacks in that a judgement of whether the measurement optical system is properly set with respect to the eye to be tested is not easy. Moreover, adjustment of the working distance in the optical axis direction is troublesome, since the adjustment of the measurement optical axis direction distance of the measurement optical system is effected by judging whether the target image is sharply formed, i.e., whether it is in focus. Although there is proposed another apparatus, in which a pair of target images are visually confirmed in the separated state when the measurement optical system is not properly set with respect to the eye to be tested, and the images are visually confirmed in an overlapping state when the system is properly set, such as system has the shortcomings that it is difficult to judge the separating amount of the pair of target images in the vicinity of the optical axis direction distance when the measurement optical system is properly set with respect to the eye to be tested (the term "vicinity" refers to a small difference between the optical axis direction distance when the measurement optical system is properly set with respect to the eye to be tested and that when the system is improperly set). At any rate, it is a difficult job to adjust the working distance by properly setting the measurement optical system with respect to the eye to be tested, and none of the conventional apparatuses are capable of performing the measurement of refractivity of an eye to be tested both promptly and accurately.